The study of bacterial cell division is important for two reasons. First, it is a basic cellular process that needs to be understood. Second, with the current scarcity of novel antibiotics, the universal and essential process of bacterial cytokinesis is increasingly relevant as a target of antimicrobial drugs. In bacteria such as Escherichia coli, cytokinesis is orchestrated by two essential and highly conserved cytoskeletal proteins: tubulin-like FtsZ and actin-like FtsA. These proteins coassemble into a circumferential polymeric structure, called the Z ring, on the inner membrane at the site of cell division. Once assembled, the ring then recruits a large complex of other proteins to the membrane, probably distributed in individual subcomplexes. This protein machine, often called the divisome, induces synthesis of septal peptidoglycan while constricting at the leading edge of the growing septum, eventually splitting the cell into two. The machine needs to be robust, yet responsive to a variety of inputs, and is therefore overbuilt. This proposal focuses on FtsA and its interactions with FtsZ and with later divisome proteins, because recent results indicate that FtsA regulates assembly of FtsZ, in addition to its role in tethering FtsZ polymers to the membrane and recruiting later divisome components. FtsZ can impart a constriction force on the membrane, but this force needs to be regulated and coordinated with growth of the division septum behind it. We hypothesize that FtsA is highly dynamic and a major regulator of FtsZ assembly and recycling, which stabilizes the machine and maximizes its flexibility. However, we do not understand what regulates FtsA's activity. Our working hypothesis is that a balance between FtsA's ability to interact with itself ad FtsZ is crucial for its activity. We propose here to elucidate the connection between FtsA's ATP binding, ATPase activity, oligomerization state, and ability to interact with and regulate dynamics of the Z ring. The complex of FtsZ, FtsA and a third protein, ZipA, constitutes the so-called proto-ring. To elucidate the relationship among proto-ring proteins, we propose to investigate the effect of mutations in FtsA that bind FtsZ more efficiently, competition between FtsA and ZipA for FtsZ binding sites, and the role of ZipA and FtsA in regulating the bundling of FtsZ protofilaments. Studies of FtsZ filament bundling will be aided by a new ftsZ mutant we recently discovered that forms excess bundles and bypasses the requirement for ZipA, and by understanding the mechanism by which a small peptide inhibits FtsZ filament bundling. Finally, we now know that FtsA, which is an early divisome protein, directly recruits a late divisome protein, FtsN, perhaps for the purpose of feedback between the growing division septum and the constricting Z ring. We propose to determine the molecular details of this interaction, how important this potentially early recruitment of FtsN is for divisome activity, and how this interaction can be circumvented.